Otherwise, If you were able to add 1 type of objective to the existing ones for this update I would seriously consider:

"Use Trait"

Use the list of traits which include:
Chaos, Undead, Bulldoze, Leech, Retaliate, Paralyzed, Webbed, Ranged, Melee, Equipment, Shatter, Flying, Mount, Undead, Boulder, etc. Basically check the trait of all of the creatures, and add them as options.

creator picks flying: make all flying creatures able to fulfill it
creator picks leech: make vampire and wizard's leech trait the winning factor
creator picks equpiment: make swords and bows the winning factor

Otherwise, you could also augment the 'survive till end' objective with exceptions or with an achille's heel.

Example:
Player 1 ---->Survive till end ------>Magic Sword
Player 1 has to survive till end, but if he's killed by a magic sword, he loses.

And you could have 2 variants of survive till end.
-one will be based on used spell, and will have the list of spells to choose from
-The other will be based on used traits, and will have the corresponding list of traits to choose from.

As attaching one of these traits can open up a variety of ways in which an objective can be fulfilled, and fit more to the game then being limited to 1 spell. This would also allow for alot of the more creative setups that demi gods could probably think of, and be relatively easy to add.

This i think would carry this feature for the meantime, while you added other components to the custom game options.

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In regard to AI objectives: I would give them both
-Defeat enemy
and
-survive till end